What is the most frictionless way to post a soundfile (preferably, but not necessarily mp3) as a status? The ultimate goal is that visitors (friends only) can listen to that sound directly within facebook, without being redirected to another facebook page or even away from facebook and without having to accept a facebook app acessing their facebook data. Preferably it should just work like the embedded youtube videos in a post.

I could of course just upload it to youtube but that way I think I couldn't keep it private to my friends and streaming the extra video data would probably cause unesessary traffic.

As an extra bonus, it would also be cool if it works (i.e. plays) within Andriod and iOS facebook app.

Facebook automatically adds an embedded Flash mp3 player, if you post a link to an mp3.

Not sure whether it will work with the Android and IOS apps, that depends if your device is flash enabled, and im pretty sure IOS does not have flash on it.

Be aware that it is illegal to post copyrighted content that you dont have the rights to.

BTW To answer your other points, youtube uses an efficient video codec, in that if you have the same image all the way through the video, the amount of data transmitted should not be much more than the size of the mp3 file, as it only transmits the image once, and just keeps that image in the window for the duration of the song.